10.1 Rigid Metal Conduit (RMC), Intermediate Metal Conduit (IMC) & Electrical Metallic Tubing (EMT)

Rigid Metal Conduit (RMC), Intermediate Metal Conduit (IMC) & Electrical Metallic Tubing (EMT)

Metallic raceways form the physical backbone of commercial and industrial electrical distribution systems. They provide mechanical protection against impact, shield conductors from physical damage, serve as listed equipment grounding conductors (EGCs) when properly installed and bonded, and contain arcing faults within heavy-gauge metal walls. On the Oklahoma Journeyman Electrician examination, questions frequently focus on distinguishing the installation rules, bending limitations, support spans, reaming procedures, and environmental restrictions governing Rigid Metal Conduit (RMC - Article 344), Intermediate Metal Conduit (IMC - Article 342), and Electrical Metallic Tubing (EMT - Article 358).

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1. Structural Comparison & Permitted Uses

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|                         METALLIC RACEWAY SPECIFICATIONS & CODE COMPARISON                               |
|                                                                                                         |
|  Feature / Parameter       RMC (Article 344)             IMC (Article 342)            EMT (Article 358)  |
|  -----------------------   --------------------------    --------------------------   ----------------- |
|  Wall Thickness            Heavy-wall steel / aluminum   Intermediate steel / alloy   Thin-wall steel/Al|
|  Joining Method            Threaded couplings / unions   Threaded couplings / unions  Set-screw/Compress|
|  Field Threadability       Standard NPT (3/4"/ft taper)  Standard NPT (3/4"/ft taper) NEVER threaded    |
|  Weight vs RMC Baseline    100% (Heaviest)               ~70% to 75% of RMC weight    ~30% to 40% of RMC|
|  Severe Physical Damage    Permitted in all locations    Permitted in all locations   NOT permitted     |
|  Direct Earth Burial       Permitted (min 6" cover)      Permitted (min 6" cover)     Supplementary prot|
|  Encased in Concrete       Permitted (all types)         Permitted (all types)        Permitted (all)   |
|  Hazardous (Classified)    Class I, Div 1 & 2            Class I, Div 1 & 2           Class I, Div 2 only
+---------------------------------------------------------------------------------------------------------+

Rigid Metal Conduit (RMC - NEC Article 344)

• Construction & Materials: RMC is a heavy-wall threaded tubing manufactured from galvanized steel, stainless steel, red brass, or aluminum (NEC 344.100). Galvanized steel RMC provides the highest level of mechanical protection recognized by the NEC.
• Permitted Uses (NEC 344.10): Permitted under all atmospheric conditions and occupancies. It may be installed exposed, concealed, in concrete, direct-buried in earth (requiring a minimum burial depth of only 6 inches per Table 300.5(A)), and in all hazardous (classified) locations (Class I, Division 1 and 2).
• Dissimilar Metals & Galvanic Corrosion (NEC 344.14 & 300.6): Where aluminum RMC is used in concrete or direct contact with earth, or where dissimilar metals come into contact in moist environments, galvanic action can rapidly corrode the metal. Aluminum RMC in concrete or direct burial must be provided with supplementary nonmetallic corrosion protection (such as factory PVC coatings or listed asphaltic tape wraps).

Intermediate Metal Conduit (IMC - NEC Article 342)

• Construction: Developed as a lighter-weight alternative to RMC, IMC uses high-strength steel alloys to provide nearly identical mechanical strength with a thinner wall thickness. IMC weighs roughly 30% less than standard RMC, making handling and overhead installation faster.
• Permitted Uses (NEC 342.10): Permitted in all atmospheric conditions and occupancies where RMC is allowed, including direct burial (6-inch cover), concrete encasement, and hazardous (classified) locations.
• Field Threading: IMC has the same outside diameter (OD) as RMC of the same trade size, allowing the use of standard conduit dies and identical threaded fittings, locknuts, and couplings.

Electrical Metallic Tubing (EMT - NEC Article 358)

• Construction: EMT (commonly called "thin-wall") is an unthreaded thin-wall metallic raceway manufactured from galvanized steel or aluminum (NEC 358.100).
• Joining Restrictions (NEC 358.28(B)): EMT shall NOT be threaded. Because the wall thickness of EMT is too thin, cutting threads into EMT severely weakens the metal and violates the listing. EMT must be joined exclusively with listed mechanical fittings (compression, set-screw, or indentation type).
• Uses NOT Permitted (NEC 358.12): EMT is prohibited:
  1. Where subject to severe physical damage (e.g., loading docks, unprotected vehicle travel paths, heavy industrial fabrication floors).
  2. For the support of luminaires or other equipment, except conduit bodies no larger than the largest trade size of the tubing (NEC 358.12 & 314.23).

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2. Maximum Number of Bends (The 360° Rule)

During conductor pulling, each bend in a conduit run creates cumulative sidewall pressure on conductor insulation. Excessive sidewall pressure causes insulation gouging, tension breaks, and short circuits.

+-----------------------------------------------------------------------------+
|                   THE 360-DEGREE CONDUIT BEND RULE                          |
|              (NEC 342.26, 344.26, 358.26, 352.26, 350.26)                 |
|                                                                             |
|   [Pull Point A]                                            [Pull Point B]  |
|     (Box/Body) --- (90°) --- (90°) --- (90°) --- (90°) --->   (Box/Body)    |
|                                                                             |
|   TOTAL DEGREES OF BEND: 90° + 90° + 90° + 90° = 360° (MAXIMUM PERMITTED)  |
|                                                                             |
|   - Bends include: 90° elbows, 45° kicks, offsets, and saddles              |
|   - Any combination totaling > 360° REQUIRES a pull box or conduit body!   |
+-----------------------------------------------------------------------------+

Code Citations & Practical Calculation:

• NEC 342.26 (IMC), 344.26 (RMC), 358.26 (EMT): "There shall not be more than the equivalent of four quarter bends (360 degrees total) between pull points, for example, conduit bodies and boxes."
• All Bends Count: The 360° limit encompasses every intentional bend, including standard 90° factory sweeps, field-bent 90° stubs, box-offset bends ($2 \times 10^\circ = 20^\circ$ or $2 \times 30^\circ = 60^\circ$), four-bend saddles ($4 \times 22.5^\circ = 90^\circ$), and 3-bend saddles ($22.5^\circ + 45^\circ + 22.5^\circ = 90^\circ$).

Worked Example: Verifying Bend Compliance

An apprentice runs a 1-inch EMT conduit from a distribution panelboard to a junction box with the following bends:

• One 90° stub-up bend leaving the panel ($90^\circ$)
• One 4-bend saddle to cross a plumbing pipe ($4 \times 22.5^\circ = 90^\circ$)
• One 90° sweep around an elevator shaft corner ($90^\circ$)
• One 45° offset into the ceiling drop ($2 \times 45^\circ = 90^\circ$)
• One box offset into the termination junction box ($2 \times 10^\circ = 20^\circ$)

$\text{Total Bend Angle} = 90^\circ + 90^\circ + 90^\circ + 90^\circ + 20^\circ = 380^\circ$

Verdict: VIOLATION (380° > 360°). The electrician must insert an intermediate pull box or conduit body (such as a Type C or Type LB conduit body) before the final offset to reset the bend count to zero.

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3. Securing and Supporting Requirements

Metallic raceways must be securely fastened in place within specified distances from terminations and supported at regular straight-run intervals to prevent vibration sagging and mechanical pullout.

+---------------------------------------------------------------------------------------------------------+
|                             SUPPORT SPACING RULES FOR METALLIC RACEWAYS                                 |
|                                                                                                         |
|  Raceway Type       Distance from Box/Fitting    Standard Support Interval    Extended Spacing Allowance|
|  ----------------   -------------------------    -------------------------    --------------------------|
|  EMT (358.30)       Within 3 ft (900 mm)         Every 10 ft (3.0 m)          Up to 5 ft from box if no |
|                                                                               structural member exists  |
|  RMC (344.30)       Within 3 ft (900 mm)         Every 10 ft (3.0 m)          Table 344.30(B)(2) allows |
|                                                                               12 ft to 20 ft straight   |
|  IMC (342.30)       Within 3 ft (900 mm)         Every 10 ft (3.0 m)          Table 342.30(B)(2) allows |
|                                                                               12 ft to 20 ft straight   |
+---------------------------------------------------------------------------------------------------------+

Support Extensions for Threaded RMC & IMC (NEC Table 344.30(B)(2) & 342.30(B)(2))

Where straight runs of RMC or IMC use threaded couplings, the maximum distance between supports is permitted to be increased based on trade size, provided the conduit is made up with threaded couplings and securely supported at each point:

Unbroken Lengths & Vertical Risers (NEC 344.30(B)(3) & 358.30(B))

• Vertical Risers in Industrial Establishments: For industrial establishments or commercial buildings with high ceilings, unbroken lengths of RMC/IMC (with threaded couplings) are permitted to be clamped at the top and bottom of the riser and supported at intervals not exceeding 20 feet (6.1 m).
• Unbroken Lengths from Fixed Equipment: For EMT and RMC/IMC, where unbroken lengths of raceway extend from industrial equipment, supports within 3 feet may be omitted if structural members are not available, provided the unbroken length is secured at the nearest accessible structural member (up to 5 feet for EMT).

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4. Field Cutting, Reaming & Threading Rules

+-----------------------------------------------------------------------------+
|                 REAMING & THREADING STANDARDS (NEC 300.4(F))                |
|                                                                             |
|   [Step 1: Square Cut]   --->   [Step 2: Ream Interior]  ---> [Step 3: Thread] |
|   Cut square using rotary       Ream using pipe reamer,       Cut standard NPT |
|   wheel cutter, bandsaw,        half-round file, or           3/4"/ft taper on |
|   or hacksaw (min 32 TPI)       reaming bit to remove burrs   RMC/IMC (NOT EMT)|
+-----------------------------------------------------------------------------+

Reaming Cut Ends (NEC 300.4(F), 342.28, 344.28, 358.28)

• Mandatory Reaming: All cut ends of conduits and tubing (RMC, IMC, EMT) must be reamed or otherwise finished to remove rough edges and burrs.
• Failure Consequences: When wire is pulled around an unreamed raceway end, sharp metal burrs act like razors, stripping thermoplastic nylon insulation (THHN) and causing line-to-ground short circuits during energization.

Field Threading Rules (NEC 342.28 & 344.28)

• Standard Tapered Threads: Field-cut threads on RMC and IMC must be cut with a standard conduit die having a 3/4-inch taper per foot (1 in 16 taper, standard NPT per ANSI/ASME B1.20.1).
• Running Threads Prohibited (NEC 342.42(B) & 344.42(B)): Running threads (straight bolt-type threads cut along the conduit body without taper) shall NOT be used on conduit for connection at couplings. Running threads weaken conduit wall strength and fail to provide a watertight, low-impedance electrical bond. Where two fixed conduits must be connected without turning the pipe, a listed three-piece union (Erickson coupling) or threadless coupling must be used.

Insulating Bushings for Conductors #4 AWG and Larger (NEC 300.4(G))

Where ungrounded conductors of size #4 AWG or larger enter a cabinet, pull box, junction box, or raceway enclosure, the conductors must be protected by a substantial, listed insulating bushing or fitting with a smooth rounded insulating surface, unless the design of the box or hub provides equivalent smooth rounded protection.

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5. Wet Location Fittings & Raceway Connectors

+-----------------------------------------------------------------------------+
|                 RACEWAY FITTINGS BY ENVIRONMENTAL LOCATION                  |
|                                                                             |
|   [Dry / Indoor Locations]           [Wet / Outdoor / Washdown Locations]   |
|   - Set-screw steel/die-cast zinc    - Gasketed rain-tight compression      |
|   - Indentation fittings             - Threaded Myers hubs with O-rings     |
|   - Standard snap-in connectors      - Threaded cast-metal conduit bodies   |
|   - Concrete-tight when taped          with neoprene gaskets                |
+-----------------------------------------------------------------------------+

Fitting Types & Code Applications:

1. Compression Fittings (Rain-Tight): Utilize an internal steel/brass gland ring and neoprene sealing ring that compresses around the EMT outer diameter when the hex nut is torqued down. Compression fittings marked "Wet Locations" or "Rain-Tight" are mandatory for EMT installed outdoors, in car washes, or exposed to weather per NEC 358.42 & 314.15.
2. Set-Screw Fittings: Utilize one or two steel set-screws that bite into the conduit wall. Listed as concrete-tight when taped, but strictly prohibited in wet locations unless specifically marked and listed for wet locations.
3. Threaded Myers Hubs: Used to connect threaded RMC or IMC to sheet metal enclosures (NEMA 3R, 4, 4X) in wet locations. Features a captive O-ring gasket and serrated bonding teeth that bite through enclosure paint to guarantee both waterproof integrity and ground-fault bonding continuity per NEC 250.92 & 314.15.
4. Drainage in Wet Locations (NEC 314.15): Enclosures, raceways, and conduit bodies installed in wet locations must be mounted to prevent moisture from entering and accumulating within the enclosure. A listed weep hole / drain opening (not larger than 1/4 inch / 6 mm) is permitted at the lowest point of the enclosure.